Microfluidic systems make it possible to manipulate small volumes of fluid, up to less than 1 microliter. They have thus opened the way to novel applications in biology, chemistry or physics that are impossible to implement successfully with conventional systems.
These systems make it possible for example to carry out analyses to the scale of individual molecules or individual cells and to perform biochemical reactions in very small volumes, greatly increasing the dynamics and reliability of the reactions: polymerisation chain reactions (PCRs) to the scale of an individual DNA molecule and new generation sequencing technologies are examples of such analyses.
On a macroscopic scale, performing purification, extraction and concentration operations by means of a solid phase is known, in particular in the context of chromatographic or immunoaffinity applications. However, using chromatographic separation microcolumns in microfluidic systems poses serious problems in terms of homogeneity of the microcolumns; furthermore, high pressures are necessary for circulating fluids in microfluidic systems, and the micromanufacture of systems with complex shapes may be laborious.
The use of magnetic particles, and in particular superparamagnetic particles, as the solid phase in microscopic-scale systems has enjoyed a certain degree of popularity. This is because magnetic particles bonded to an analyte of interest may be retained by a magnet while the surrounding fluid is eliminated. Methods of this type may be multiplexed, for example using multiple magnets disposed at the bottom of microtitration plates. Such systems do however have the drawback of limitations for example vis-à-vis the reaction speed, relatively high necessary elution volumes and a mediocre efficacy of mixing and rinsing.
Using superparamagnetic particles in microfluidic systems may make it possible to solve these problems: this is because the surface area/volume ratio is then high and the possibilities of functionalisation of the surfaces are numerous, while the magnetic properties of the particles afford easy contactless manipulation and make it possible to form compact structures such as microcolumns.
One challenge posed by methods of detection on chips is that of the concentration of the analytes in the analysis volume, or on the detection surface, to a level that must be sufficiently high vis-à-vis the detection threshold. It is therefore desirable to concentrate the samples before detection, which is tricky to achieve. This challenge is particularly great in the case of diagnostic applications, in which biological markers may be present at a very low concentration. For a typical microfluidic system functioning with a sample volume of 1 microliter or less, a step of pre-concentration from a larger volume (for example several milliliters) may be necessary.
Document WO 98/23379 describes a device for separating particles or molecules by migration through a ferrofluid. A magnetic field is applied perpendicular to the direction of movement of species to be separated, in order to create regions rich in or depleted of magnetic particles.
Document EP 1331035 describes an apparatus for retaining magnetic particles in a fluid-circulation cell. The cell is placed between the poles of a magnet, and high local gradients of a magnetic field are generated by means of microstructures present at the surface of the poles of the magnet. It is these high gradients that immobilise the magnetic particles.
Document EP 1974821 describes a system in which magnetic particles can be moved along a channel by a succession of electromagnets facing each other on either side of the lateral walls of the channel.
Document U.S. Pat. No. 7,309,439 describes a device for transporting magnetic particles in a capillary tube by means of magnetic devices placed around the tube.
The article by Beyor et al. in Biomed. Microdevices 10:909-917 (2008) describes a system using magnetic particles for detecting pathogenic agents on chips. A movable magnet is placed under a microchannel comprising bifurcations, which creates a compact barrier of particles that can be moved.
The article by Gijs et al. in Chem. Rev. 110:1518 (2010) is a review of the microfluidic applications of magnetic particles for biological analysis and catalysis. In particular, it is disclosed that magnetic particles can be retained and manipulated in microsystems by means of fixed or movable magnets or electromagnets placed on one side of a channel (below) or facing each other on either side of the channel (above and below).
Document WO 2010/041231 also describes a system in which magnetic particles are immobilised by means of a magnetic field transverse to the direction of flow.
Document WO 2010/041230 describes a microfluidic device for detecting analytes. The device comprises a microchannel and magnets disposed on either side of the microchannel and oriented so that a magnetic field essentially colinear with the direction of flow in the microchannel is generated. This system also makes it possible to create a plug of magnetic particles in a region of the microchannel. It is not suitable for use at relatively high rates.
Finally, it should be noted that systems proposing a circulation of fluid through combinations of magnetic and non-magnetic particles have been proposed.
Thus the article by Seibert et al. in Biotechnol. Prog. 14:749-755 (1998) describes a macroscopic system in which a series of annular coils are placed around a tube containing a fluidised bed for fermentation, containing magnetic and non-magnetic particles. The magnetic particles reduce the mixing effects in the bed.
The article by Tong et al. in Biotechnol. Prog. 19:1721-1727 (2003) describes another macroscopic system comprising a single coil around a fluidised bed creating a transverse field, which is used for increasing the compactness of the bed.
However, the latter two systems do not make it possible to process small volumes of fluid.
In summary of the above, the prior art proposes two types of microfluidic systems comprising magnetic particles. In the first type, the particles are organised in low-density static chains. The flow rate of fluid in this type of system may be relatively high because of the low density of the magnetic particles. However, the time taken for transporting the species of interest to the magnetic particles is high because of the large distances to be travelled. In the second type, the particles are organised in compact blocks, with generally regions where fluid circulates in the vicinity of the compact blocks. These compact blocks form plugs vis-à-vis the flow, so that the flow rate is limited.
There therefore exists a need to overcome the drawbacks of the systems of the prior art and in particular to have available a microfluidic system functioning at a relatively high flow rate, while ensuring good contact between the species of interest and the magnetic particles, that is to say a short time for the species of interest to diffuse towards the magnetic particles.